A large international team of astronomers has used NASA’s Hubble Space Telescope to learn more about so-called quenched galaxies – galaxies that are no longer forming stars.
This image shows 20 of the quenched galaxies (NASA / ESA / M. Carollo, ETH Zurich).
Quenched galaxies in the distant past appear to be much smaller than the quenched galaxies in our modern-day Universe. This has always puzzled astronomers — how can these galaxies grow if they are no longer forming stars?
As these galaxies are no longer forming new stars, they were thought to grow by colliding and merging with other smaller quenched galaxies some five to ten times less massive. However, these mergers would require many such small galaxies floating around for the quenched population to snack on — which we do not see.
Until recently it had not been possible to explore a sufficient number of quenched galaxies, but now the astronomers led by Dr Marcella Carollo of ETH Zurich, Switzerland, have identified and count these galaxies throughout the last eight billion years of cosmic history.
Dr Carollo and her colleagues used the large set of Hubble images, alongside additional observations from the Canada–France–Hawaii Telescope and the Subaru Telescope, to peer back to when the Universe was less than half its present age. These observations mapped an area in the sky almost nine times that of the full Moon.
The quenched galaxies seen at these times are small and compact – and surprisingly, it seems they stay that way. Rather than puffing up and growing via mergers over time, these small galaxies mostly keep the size they had when their star formation switched off.
“We found that a large number of the bigger galaxies instead switch off at later times, joining their smaller quenched siblings and giving the mistaken impression of individual galaxy growth over time,” said Dr Simon Lilly, also of ETH Zurich, co-author of a paper reporting the findings in the Astrophysical Journal (full paper in .pdf).
“It’s like saying that the increase in the average apartment size in a city is not due to the addition of new rooms to old buildings, but rather to the construction of new, larger apartments,” said study co-author Dr Alvio Renzini of INAF Padua Observatory, Italy.
“This tells us a lot about how galaxies have evolved over the last eight billion years of the Universe’s history. It was already known that actively star-forming galaxies were smaller in the early Universe, explaining why they were smaller when their star formation first switched off.”
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Bibliographic information: C. M. Carollo et al. 2013. Newly Quenched Galaxies as the Cause for the Apparent Evolution in Average Size of the Population. ApJ 773, 112; doi: 10.1088/0004-637X/773/2/112
